add support for multiline string in headers

Some languages (such as Polish, Russian or Romanian) do have more
complicated plural forms. Those are still expressible by a more
complicated mathematical expression. However, the msgmerge tool of
gettext will in these cases write multiline header fields. When parsing
such files with this lib, the headers get screwed up, so this patch
provides an example (from a pl_PL po file) and fixes this by joining the
lines in the header, before doing the actual parsing.